Roof Ventilation

Why is there a need for roof ventilation?

As modern construction techniques continue to place more emphasis on energy conservation, minimising the heat loss from buildings, insulation thicknesses have increased and air tightness is constantly improving. Although this is a way of achieving good energy performance certification, there is now far less opportunity for water vapour to be dissipated through fortuitous ventilation in the property, and therefore a higher chance of condensation build up and potential damage.

Few new buildings contain open fireplaces, single glazed windows and solid floor construction has become the norm. The increase in levels of moisture being created has gone hand in hand with more heating and greater insulation levels creating greater temperature differentials within the fabric of the building.

Water vapour in the air causes a vapour pressure, the warmer the air, the greater capacity there is to contain moisture. The greater the moisture content, the higher the vapour pressure becomes.  This vapour pressure acts in all directions and will cause the water vapour to pass through the smallest of gaps to anywhere where the vapour pressure is lower until equilibrium is achieved.

Pitched, cold and warm roof ventilation

First of all, what type of construction is the roof? You may have heard of the terms cold roof and warm roof construction, but what do these actually mean?

In pitched roof construction, the insulation can be placed horizontally above the ceiling level forming a ‘cold roof’; it can be inclined above, between or below the rafters forming a ‘warm’ or ‘hybrid’ roof, or positioned in the roof structure in different ways where a ‘room-in-roof’ is to be created that can result in warm and cold roof construction types. Cold roof voids are generally always required to be ventilated; however, where vapour permeable (or Low Resistance, LR) underlays are specified, the levels required may be reduced. The enclosed void of a warm pitched roof may not require to be ventilated, but this is usually subject to the requirement to install a fully sealed Air and Vapour Control Layer (AVCL) in conjunction with an air-open roof covering.

Understanding the roof construction of any new build or refurbishment is essential to determine what products are required for the job.

Ventilation & Building Regulations

Approved Document C2 requires that roofs be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation. This requirement will be met if the roof is designed and constructed in accordance with Annex H of BS 5250 ‘Code of practice for control of condensation in buildings’ and BS EN ISO13788; ‘Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods.’ Further guidance is given in BRE Report BR262 ‘Thermal insulation: avoiding risks’.

To avoid excessive moisture transfer into roof voids, gaps and penetrations for pipes and electrical wiring should be filled and sealed, particularly in areas of high humidity such as kitchens and bathrooms and an effective draught seal should be provided to loft hatches to reduce the inflow of warm air and moisture. Vapour control layers can reduce the amount of vapour entering roof voids but cannot be relied on as an alternative to ventilation. A complete barrier to moisture is needed for this.

Scottish Technical Handbooks section 3.15 requires that dwellings shall be so constructed as to protect the building and its users, so far as may be reasonably practicable, from harmful effects caused by surface and interstitial condensation. Both these requirements are deemed to be satisfied by following the guidance given in BS 5250.

BS 5534; Slating and Tiling for Pitched Roofs and Vertical Cladding – Code of Practice recommends that roof ventilation be provided in accordance with BS 5250.

Roof Ventilation Installation

To provide ventilation, opening points can usually be provided on the following areas of a pitched roof:

• Roof surface
• Ridge line
• Eaves, soffits and fascias

For each of these areas, there are numerous products to suit different applications and construction details. They are also vital to meet the appropriate requirements and regulations by providing adequate ventilation levels into the roofs space. Our expansive range of ventilation products can deliver the ideal solution to these common issues.

Our independently tested flush fitting tile and slate vents provide some of the highest ventilation levels on the market in addition to being used for soil pipe ventilation and as terminals for mechanical extraction. Our universal dry fix ridge and hip system provides for full mechanical fixing of tiles while being compliant with BS 5534 and BS 5250.

For more information on the requirements for roofing ventilation, take a look at our Back to Basics article which can be read on Roofing Today.